1. Field of the Invention
The present invention relates to a variable gear ratio hydraulic power steering device with a failsafe function for a variable ratio transmission mechanism.
2. Discussion of the Related Art
Heretofore, as hydraulic power steering devices, there has been known one for example which is provided with a variable ratio transmission mechanism for varying the rotation transmission ratio between a steering shaft coupled to a steering handle and a pinion shaft coupled to a pinion gear meshing with a rack gear and which is constructed to make variable the overall gear ratio (a value obtained by dividing the steering angle of a steering handle by the steered angle of steerable wheels) of a steering mechanical train thereof (e.g., refer to Patent Document 1). With the variable ratio transmission mechanism, it can be realized to set the overall gear ratio to be low (i.e., to a quick side) in a low speed range for reduction of the handle manipulation amount by the driver and to set the overall gear ratio to be high (i.e., to a slow side) in a high speed range for enhancement of the traveling stability.
Further, there has been known one which is provided with a flow volume control valve for controlling the flow volume of operating fluid supplied from an oil pump to a power cylinder and which is constructed to control the flow volume control valve electronically (e.g., refer to Patent Document 2). In the power steering device, a vehicle speed, a handle steering angle and so on are taken as control inputs, and the discharge flow volume of the oil pump is regulated by controlling the valve opening degree of the flow volume control valve based on these control inputs. Thus, steering assist force to be generated is properly controlled by regulating the discharge flow volume of the oil pump, so that the handle steering feeling felt by the driver and energy-saving effect are compatible with each other.
Further, there has been known a variable gear ratio hydraulic power steering device for example which is constituted by combining the aforementioned flow volume control valve for controlling the discharge flow volume of the oil pump with the aforementioned variable ratio transmission mechanism for varying the rotation transmission ratio. In the variable gear ratio hydraulic power steering device, the control by the variable ratio transmission mechanism is cooperated with the control of the discharge flow volume by the flow volume control valve, whereby further improvements are made in handle steering feeling as well as in energy-saving effect.                [Patent Document]: Japanese unexamined, published patent application No. 10-324263        [Patent Document]: Japanese examined, published patent application No. 2-56274        
However, the variable gear ratio hydraulic power steering device in the prior art is accompanied by the following problems. That is, the flow volume of the operating fluid discharged by the aforementioned oil pump is set on the assumption that the aforementioned overall gear ratio is being controlled variably. Thus, when the rotational transmission ratio of the variable ratio transmission mechanism is fixed for some troubles, it tends to occur that the flow volume of the operating fluid discharged by the oil pump falls into an excessive or short state. Further, it tends to occur that the excessive or short flow volume discharged from the oil pump causes the steering assist force to be excessive or insufficient, whereby the handle steering feeling, the traveling stability or the like cannot be kept in a proper state.